A technique for estimating the inside of an object is important. Examples of the application of the technique include detecting a foreign object, such as a needle, contained in food, and estimating the shape of a foreign object, such as a needle, stuck in the skin. High-accuracy estimation of the inside of an object supports an increase in food safety and medical advancement.
As a method of estimating the inside of an object, a method is widely used which is based on X-ray computed tomography. In the method which uses X-ray computed tomography, the following procedure is basically performed. First, X-rays are emitted into an object which contains a foreign object and the amount of the X-rays exiting the object is observed. Subsequently, the distribution of absorptance of light (X-rays) in the object is estimated based on the ratio of the amount of outgoing X-rays to the amount of incident X-rays (that is, based on the amount of absorption of light). In practice, X-rays are directed to an object from 360-degree direction and the amount of attenuation of X-rays exiting the object is observed. Subsequently, the distribution of absorptance of the inside of the object is estimated using a method referred to as a back projection or a filtered back projection.
By emitting light beams from a light source, observing the outgoing amount of the light beams transmitted through the object, and analyzing the observation results in such a manner, it is possible to estimate the positions and the shapes of absorbing objects in the object.